This disclosure relates in general to multiphase flow measurement for oil-gas wells and, but not by way of limitation, to high gas volume flow fraction/holdup and/or velocity/flow-rate measurements.
Most oil-gas wells ultimately produce both oil and gas from an earth formation, and also often produce water. Consequently, multiphase flow is common in oil-gas wells. Surface monitoring of oil and gas producing wells is tending towards metering multiphase flows with a wide range of gas volume fraction (GVF). An example of this is the so called wet-gas wells, where the GVF is typically larger than 95% and the liquid flow rate is typically no more than a few hundred barrels per day. For such production pipes, it is often required to measure the gas flow rate and the liquid flow rate, as well as the composition of the liquid phase, e.g. water/liquid hydrocarbon ratio (WLR). For wells with a GVF<95%, in-line multiphase flow meters have been/are often used.
Two existing approaches to metering high GVF flows are separation and mixing. The separation approach provides for splitting the flow into an almost liquid flow in one channel/conduit and an almost gas flow in a separate channel/conduit and then separately metering the separated flows using single-phase flow meters. The mixing approach attempts to minimize the slip between the different phases by mixing the phases into a homogeneous mixture so that the velocity and holdup measurements can be simplified.
The existing methods are largely capable of providing good accuracy for metering gas flows with high GVF, however, the liquid rate metering accuracy is relatively poor. The disadvantage of such methods also include increased cost associated with the separation and mixing devices and extra pressure drop in the pipeline and/or disruption to the flow in the pipeline resulting from the introduction of the separation and/or mixing devices into the pipeline. Additionally, at high GVF, the mixing method may not provide for accurately measuring the holdup and WLR because the liquid holdup is very low under such conditions.